Data originating from a number of independent sources and sent via a plurality of data channels at various independent bit rates cannot be readily transmitted or recorded utilizing synchronous data transmission or recording devices. The use of such synchronous devices is much preferred over asynchronous transmission or recording because each asynchronous channel needs a separately tuned bit sync, and a separate deskew and error correction circuit to be utilized upon reception or reproduction of the transmitted data.
Prior art devices solved the above-indicated problem by sampling the asynchronous data synchronously on all channels utilizing a common sampling frequency. The thusly obtained samples were then transmitted or recorded utilizing synchronous data transmission or recording techniques, respectively. There are significant disadvantages associated with the foregoing technique as follows. In accordance with well known Nyquist's sampling theorem, also referred to as the Nyquist criteria, the sampling signal frequency must be greater than twice the highest bit rate of the sampled digital signal. Otherwise the signal recovered from the samples will not be an exact replica of the original input signal but will suffer undesirable distortion and information loss. Another disadvantage is that utilizing a common sampling clock frequency for a wide range of input signal frequencies results in inefficient use of the data transmission or recording channel bandwidth, particularly for relatively low data rates. For example, when the input data rate varies from 1 to 100 kilobits per second (kbps), and the common sampling clock has a frequency of 250 kbps, there will be two and one-half samples provided during each cycle of the 100 kbps signal, and 250 samples during each cycle of the 1 kbps signal. It is seen from the foregoing example that while there is utilized a near optimum number of samples per bit of the high bit rate signal, the large number of samples per bit of the low frequency signal is wasteful of the transmission time or recording medium, respectively, and results in a highly inefficient use of the transmission or recording channel bandwidth. Another disadvantage related to sampling of asynchronously clocked input data is the reduction of maximum bandwith which would be otherwise allowable for data transmission, due to frequency limitations of the data transmission or recording equipment.